• Main Extension

    The addition of pipe to an existing gas main to serve new customers.

  • Main, Gas

    Pipe used to carry natural gas from one point to another. As contrasted with service gas pipes, mains usually carry natural gas in large volume for general or collective use. See PIPELINE SYSTEM.

  • Mainline


  • Maintenance Derating

    The removal of a component for scheduled repairs that can be deferred beyond the end of the next weekend, but requires a reduction of capacity before the next planned outage.

  • Maintenance Outage

    The removal of a unit from service to perform work on specific components that can be deferred beyond the end of the next weekend, but requires the unit be removed from service before the next planned outage.

  • Margin

    Money that buyers and sellers of futures and exchange-traded options must put up with the clearinghouse to assure performance on the contracts. For over-the-counter options, margins are negotiated between the counterparties. In both cases, the amount of margin required varies with the price fluctuations of the underlying contract. Open positions are marked-to-market daily and, in times of extreme volatility, marked-to-market intra-day as well.

  • Marginal Cost

    The increase or decrease in total costs brought about by a one unit increase or decrease in output.

  • Marginal Cost Pricing

    A system of pricing designed to ignore all costs except those associated with producing the next increment of production. Sometimes referred to as incremental cost pricing.

  • Marginal Price of Power

    Price for power from a unit which is already running. Includes only fuel and variable O&M plus a markup. (System Lambda + markup.)

  • Marked-to-Market

    The readjustment in the value of a derivative instrument or product position with respect to the current market value of its underlying instrument.

  • Market-Based Sales Rates

    Sales rates resulting from arm's length negotiations, rather than the pipeline's or its affiliate's actual costs of supplies.

  • Market Center

    See HUB.

  • Market-Clearing Level

    The maximum price at which all of an available commodity (natural gas or electricity) can be sold in a specified market.

  • Market Makers

    Various commercial and investment banks that make a market in swaps and hold significant portfolios of swap contracts. A market participant who provides liquidity to the markets by continuously quoting prices at which he/she will buy or sell a particular instrument.

  • Market-Out

    A provision in an energy sales agreement that allows one or both parties to demand re-negotiation of the sales price and/or terminate the contract if the contract sales price no longer reasonably reflects the current market.

  • Market Price

    The current price of an underlying instrument in the marketplace.

  • Market Risk

    The exposure that results from holding an unhedged swap as market conditions change.

  • Marketable Gas


  • Marketer

    An entity engaged in bringing together sellers and buyers, usually on a spot-market basis, assisting in negotiations, and arranging transportation and delivery terms.

  • Marketing Affiliate

    Marketer owned or substantially controlled by an affiliate gas pipeline or electric utility.

  • Master Metering

    Separate metering of individual units in a new building is required if the occupant has control over energy use in the unit and if benefits exceed costs.

  • Matched Book

    A situation where a market maker has arranged exactly offsetting swap transactions so that there is no net market risk.

  • Maximum Demand

    The greatest of all demands of the load that has occurred within a specified period of time.

  • Mcf

    One thousand cubic feet of natural gas.

  • Mean Temperature

    As used by the Weather Bureau in determining degree days, the average of the maximum and the minimum dry-bulb atmospheric temperatures in degrees Fahrenheit recorded for each day.

  • Megawatt-hour (MWh)

    One million watt-hours of electric energy. A unit of electrical energy which equals one megawatt of power used for one hour.

  • Megawatt (MW)

    A unit of electrical power equal to one million watts or one thousand kilowatts.

  • Melded Rate

    A rate which reflects the combined costs of different sources of power. Typically costs of existing hydroelectric projects and costs of newer thermal plants are said to be melded when combined or averaged together in one rate.

  • Merit Dispatch

    See Economic Dispatch. The process of dispatching generator units in order of ascending marginal costs of production.

  • Metallurgical Coal

    Coal, typically bituminous, which is heated in a substoichometric atmosphere to the extent that the volatile matter of the coal is released and the coal passes through the "plastic stage" to become metallurgical coke, which is used as the carbon input in an integrated steel mill.

  • Meter, Diaphragm

    A gas meter in which gas passes through two or more chambers and moves diaphragms geared to a volume-indicating dial.

  • Meter, Five Light

    The smallest size diaphragm meter, usually installed for a domestic consumer. The standard capacity is approximately 150 cubic feet per hour. Recently a three light meter has also come into frequent use.

  • Meter, Gas

    An instrument for measuring and indicating, or recording, the volume of natural gas that has passed through it.

  • Meter, Orifice

    A meter for measuring flow of fluid through a pipe or duct by measurement of the pressure differential across a plate that has a precisely cut hole in its center.

  • Meter, Rotary Displacement

    A positive-pressure blower used as a meter in which gas pressure turns the blower and the volume of gas passing through is proportionate to the number of revolutions.

  • Meter, Venturi

    A fluid-flow meter in which the fluid flow is determined by measuring the pressure drop caused by the flow of the fluid through a Venturi throat or tube. The pressure drop across the tube is proportionate to the fluid-flow rate.

  • Metered Value

    A measured electrical quantity that may be collected by telemetering, SCADA (remote control and telemetry used to monitor system) or other means.

  • Metering

    Use of devices that measure and register the amount and/or direction of energy quantities relative to time.

  • Methane (CH 4)

    The lightest in the paraffin series of hydrocarbons. It is colorless, odorless and flammable; the major portion of marsh gas and natural gas.

  • Mill

    One-tenth of one cent. A common unit for pricing electricity.

  • Min-Max

    See Collar.

  • Minimum Charge (Minimum Bill Clause)

    A clause in a contract that provides that the charge for a prescribed period shall not be less than a specified amount.

  • Minimum Commodity Bill

    A charge that requires the purchaser to pay up to the full charge for a specified percentage of contracted amounts whether or not the specified amount of service is actually taken.

  • MMBtu

    One million British thermal units.

  • MMcf

    One million cubic feet of natural gas.

  • Model Conservation Standards

    Construction standards for new electrically heated residential and commercial structures, and conversion standards for residential and commercial structures that to electric space heating.

  • Modified Fixed-Variable Rate Design

    A rate-design methodology employed by the FERC for interstate natural gas pipelines that allocates all fixed costs except return on equity and related taxes to the demand charge, and that allocates return on equity and related taxes, all production and gathering .costs, and all variable costs to the commodity charge.

  • Money Market Swap

    Swaps with relatively short maturities, generally less than three years.

  • Monopoly

    A state of exclusive or near-exclusive ownership or control of a commodity, service or facility through legal privilege, command of supply, or concerted action, making possible the manipulation of prices.

  • Monopsony

    In contrast to monopoly, monopsony is a market condition in which there are a large number of sellers and only one buyer.

  • Most Favored Nation Clause

    A contract clause that ties the contract price to the rates paid in other contracts, usually specifying the region to be taken into consideration, such as a county, state, field, basin, or other geographic or geologic area. Generally, most favored nation clauses require that the other contracts be recent in time and for like quantity, quality and contract term.

  • Mothball

    To place a generating facility in an inactive state so that it can neither be brought into operation immediately nor counted towards reserve margin.

  • Municipal Utility

    A utility owned and operated by a municipality or group of municipalities.

  • Municipalization

    The process by which a municipal entity assumes responsibility for supplying utility service to its constituents. In supplying electricity, the municipality may generate and distribute the power or purchase wholesale power from other generators and distribute it.

  • Municipalization-Lite or Muni-Lite

    A municipal utility that owns only meters and purchases power from the wholesale market without undertaking the bur-den of acquiring significant transmission or distribution facilities.

  • Must Run Units

    A specific generating unit that has been designated by the system operator to be on line or on the grid to insure the flow of electricity. This must run unit is outside of economic dispatch and mayor may not be a system's most efficient unit. A unit may be designated as must run for operating reasons that may include system reliability, voltage control, or system stability.